/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package com.feilong.lib.io;

import java.nio.charset.Charset;
import java.nio.charset.StandardCharsets;
import java.util.Collections;
import java.util.SortedMap;
import java.util.TreeMap;

/**
 * Charsets required of every implementation of the Java platform.
 *
 * From the Java documentation <a href="https://docs.oracle.com/javase/7/docs/api/java/nio/charset/Charset.html">
 * Standard charsets</a>:
 * <p>
 * <cite>Every implementation of the Java platform is required to support the following character encodings. Consult
 * the release documentation for your implementation to see if any other encodings are supported. Consult the release
 * documentation for your implementation to see if any other encodings are supported. </cite>
 * </p>
 *
 * <ul>
 * <li><code>US-ASCII</code><br>
 * Seven-bit ASCII, a.k.a. ISO646-US, a.k.a. the Basic Latin block of the Unicode character set.</li>
 * <li><code>ISO-8859-1</code><br>
 * ISO Latin Alphabet No. 1, a.k.a. ISO-LATIN-1.</li>
 * <li><code>UTF-8</code><br>
 * Eight-bit Unicode Transformation Format.</li>
 * <li><code>UTF-16BE</code><br>
 * Sixteen-bit Unicode Transformation Format, big-endian byte order.</li>
 * <li><code>UTF-16LE</code><br>
 * Sixteen-bit Unicode Transformation Format, little-endian byte order.</li>
 * <li><code>UTF-16</code><br>
 * Sixteen-bit Unicode Transformation Format, byte order specified by a mandatory initial byte-order mark (either order
 * accepted on input, big-endian used on output.)</li>
 * </ul>
 *
 * @see <a href="https://docs.oracle.com/javase/7/docs/api/java/nio/charset/Charset.html">Standard charsets</a>
 * @since 2.3
 * @version $Id$
 */
public class Charsets{
    //
    // This class should only contain Charset instances for required encodings. This guarantees that it will load
    // correctly and without delay on all Java platforms.
    //

    /**
     * Constructs a sorted map from canonical charset names to charset objects required of every implementation of the
     * Java platform.
     * <p>
     * From the Java documentation <a href="https://docs.oracle.com/javase/7/docs/api/java/nio/charset/Charset.html">
     * Standard charsets</a>:
     * </p>
     *
     * @return An immutable, case-insensitive map from canonical charset names to charset objects.
     * @see Charset#availableCharsets()
     * @since 2.5
     */
    public static SortedMap<String, Charset> requiredCharsets(){
        // maybe cache?
        final TreeMap<String, Charset> m = new TreeMap<>(String.CASE_INSENSITIVE_ORDER);
        m.put(StandardCharsets.ISO_8859_1.name(), StandardCharsets.ISO_8859_1);
        m.put(StandardCharsets.US_ASCII.name(), StandardCharsets.US_ASCII);
        m.put(StandardCharsets.UTF_16.name(), StandardCharsets.UTF_16);
        m.put(StandardCharsets.UTF_16BE.name(), StandardCharsets.UTF_16BE);
        m.put(StandardCharsets.UTF_16LE.name(), StandardCharsets.UTF_16LE);
        m.put(StandardCharsets.UTF_8.name(), StandardCharsets.UTF_8);
        return Collections.unmodifiableSortedMap(m);
    }

    /**
     * Returns the given Charset or the default Charset if the given Charset is null.
     *
     * @param charset
     *            A charset or null.
     * @return the given Charset or the default Charset if the given Charset is null
     */
    public static Charset toCharset(final Charset charset){
        return charset == null ? Charset.defaultCharset() : charset;
    }

    /**
     * Returns a Charset for the named charset. If the name is null, return the default Charset.
     *
     * @param charset
     *            The name of the requested charset, may be null.
     * @return a Charset for the named charset
     * @throws java.nio.charset.UnsupportedCharsetException
     *             If the named charset is unavailable
     */
    public static Charset toCharset(final String charset){
        return charset == null ? Charset.defaultCharset() : Charset.forName(charset);
    }
}
